As semiconductor wafer processing machines move toward more automation, cassettes are used to move the wafers from one machine to another. The cassettes are made of soft plastic to avoid damage to the wafers. Such cassettes, however, are subject to warping and deformation in use and are not dimensionally stable. Sometimes wafers are broken or otherwise removed from the group of wafers in the cassette, thereby creating a blank space in the cassette. Valuable processing time is thereby wasted.